Light sources of three colors: R (red), G (green), and B (blue) are required as light sources used for a device that displays a color image, such as a projector device or a projection TV. In recent years, as these light sources, wavelength conversion laser devices (laser oscillators) that oscillate laser light in a 900 nm band, laser light in a 1 micrometer band, and laser light in a 1.3 micrometer band as fundamental laser light, and that convert (SHG, Second Harmonic Generation) each fundamental laser light by using a nonlinear material into a second harmonic wave have been developed.
As an example of a wavelength conversion laser device, there has been provided a wavelength conversion laser device that consists of a semiconductor laser element, a solid-state laser element, and a wavelength conversion element (refer to patent reference 1). In this wavelength conversion laser device, the solid-state laser element absorbs pumping light generated by the semiconductor laser element to generate a fundamental wave of laser light, and the wavelength conversion element converts the wavelength of the fundamental wave generated by the solid-state laser element to generate a second harmonic wave. The three elements in this wavelength conversion laser device are produced individually, and alignment is performed in such a way that their optical axes are aligned with one another. Further, a coating having optimal reflectivity for each of the fundamental wave and the second harmonic wave is applied to each of front and rear end surfaces of each of the elements.
In a case in which the solid-state laser element and the wavelength conversion element are integral with each other with joining, the coating on a joining plane side of each of the elements becomes unnecessary, and the alignment between the elements also becomes unnecessary, and therefore it becomes possible to provide an improvement in the ease of production, a cost reduction, etc.
FIG. 16 is a block diagram showing a wavelength conversion laser device in which a solid-state laser element and a wavelength conversion element are integral with each other with joining. In the wavelength conversion laser device shown in FIG. 16, the solid-state laser element 103 and the wavelength conversion element 104 are arranged in front of a semiconductor laser element 101 that generates pumping light, and the solid-state laser element 103 and the wavelength conversion element 104 are secured onto a heat sink 102 for cooling. An end surface 103a and an end surface 103b are formed on the solid-state laser element 103, an end surface 104a and an end surface 104b are formed on the wavelength conversion element 104, and the end surface 103b of the solid-state laser element 103 and the end surface 104b of the wavelength conversion element 104 are joined to each other.
The end surface 103a of the solid-state laser element 103 has a reflecting film that allows the pumping light emitted from the semiconductor laser element 101 to pass therethrough, and that total-reflects a fundamental wave of laser light generated by the solid-state laser element 103. In contrast, the end surface 104b of the wavelength conversion element 104 has an optical film that reflects the fundamental wave and that allows a second harmonic wave of laser light to pass therethrough. Each of these total reflection film, antireflection film, and optical film consists of, for example, dielectric thin films which are laminated.
As a method of joining the solid-state laser element 103 and the wavelength conversion element 104 to each other, a method of optically joining them to each other by using an optical contact, diffusion bonding, or surface activated bonding is chosen in many cases. As a method of positioning the semiconductor laser element 101, an active alignment method of adjusting and fixing the position of the semiconductor laser element 101 in such away that the light intensity of the laser light outputted from the wavelength conversion element 104 is maximized when the pumping light is emitted from the semiconductor laser element 101 is used typically.
When the laser light emitted from the semiconductor laser element 101 is incident upon the solid-state laser element 103, activity ions are pumped within the solid-state laser element 103, and, as a result, a fundamental wave laser-oscillates. At this time, the reflecting surfaces (resonance surfaces) that construct the resonator of the fundamental wave are the end surface 103a which is the rear end surface of the solid-state laser element 103 and the end surface 104b which is the front end surface of the wavelength conversion element 104. Hereafter, a case in which the fundamental wave traveling in a direction of A shown in the figure is reflected by the end surface 103b or the end surface 104a, or a case in which the fundamental wave traveling in a direction of B shown in the figure is reflected by the end surface 103b or the end surface 104a will be considered. In this case, because the reflected wave is usually not in phase with the fundamental wave, the reflected wave results in an optical loss without contributing to the oscillation. More specifically, when reflection occurs at the end surface 103b or the end surface 104a, the light density of the fundamental wave generated by the solid-state laser element 103 drops and hence the optical power characteristics of the second harmonic wave get worse.
Generally, because the thermal expansion coefficient of the solid-state laser element 103 does not completely match that of the wavelength conversion element 104, the joining between them may unstick due to generation of heat at the time of assembling them and heat caused by operation, and a gap may occur between them. In the conventional laser device, there is a case in which even when this gap is very narrow, the reflection increases according to the refractive index difference between each element and the gap (air), and the optical power characteristics get worse.
Hereafter, a case in which the oscillation wavelength of the semiconductor laser element 101 is 808 nm, Nd:YVO4 (Nd-doped yttrium vanadate crystal) is used as the solid-state laser element 103, and PPLN (Periodically Poled Lithium Niobate) is used as the wavelength conversion element 104 will be considered. In this case, the solid-state laser element 103 generates a fundamental wave having a wavelength of 1,064 nm by using the pumping light emitted from the semiconductor laser element 101.
FIGS. 17 and 18 show the result of simulation of determining the reflectivity when changing the width of the gap between the solid-state laser element 103 and the wavelength conversion element 104 in consideration of multipath reflection of an electric field by using the Fresnel formulae. Because the polarization of the fundamental wave generated by the solid-state laser element 103 is made to be P in many cases, a P wave will be considered hereafter. Although it is necessary to make the reflectivity at the joining portion between the solid-state laser element 103 and the wavelength conversion element 104 be about 0.6% or less in order to provide adequate optical power characteristics, the gap width permitted at this time is very as small as 16 nm or less. In an actual element, the gap width easily exceeds this permitted gap width, and, as a result, the optical power characteristics get worse. In addition, because increase of the gap width reduces the difference between the reflectivity of the P wave and that of the S wave, an oscillation (parasitic oscillation) of the S wave which does not contribute to the wavelength conversion occurs easily.